would not to have responded over the pulpit to the, perhaps many, questions he had been asked about attending BYU. If he had been approached one on one and asked that question, wisdom on his part would have been to dance around the answer to avoid hurting anyone's opinion on their school while still not answering the question. Knowledgeable people would be able to determine that he is trying to avoid being put in the corner and that he was starting to look uncomfortable and drop the line of questioning.
If he had been pinned to the wall on a one on one basis, and asked that question, he should respond honestly to the question with a little bit of sugar on it. Yet he is still entitled to his opinion while serving a mission. As a missionary he should be and it sounded like he was, honest with his answer. I love to have the missionaries in my home for the spirit that they bring with them and am willing to overlook all their flaws in order to just enjoy them.
If you don't think you'll like the answer....don't ask the question.
He chose unwisely for his forum to answer the questions he had been getting, and it sounds like an overreaction on your part to be so hypersensitive to a perceived slight at your Alma Mater. Only in Utah county would alumni get steamed about a situation like this.